what HARDWARE should we upgrade in order to boost ArcSDE performance

Say, if we have extra budget to do some hardware upgrading, what hardware should we upgrade in order to boost ArcSDE performance? ie. CPU cores, CPU speed, memory size, memory speed or video card? Which hardware is the most important key to affect ArcSDE performance?

system info:
Windows 2008 R2
Oracle 11.2.0.1
ArcSDE 9.3.1 sp2

Thanks.